The present invention relates to high-strength films prepared from aqueous dispersions of block copolymers of vinyl aromatic monomers and conjugated dienes wherein the conjugated diene block is hydrogenated.
Block copolymers of the conventional A--B--A type form strong films when cast from solutions in organic solvents. The use of aqueous dispersions or latices to form films or articles of intricate design is preferred to the use of casting from solutions because no objectionable fumes are released during the drying step. However, films of comparable thickness prepared by casting from their aqueous dispersions or latices are generally weak.
To improve the strength of such films, U.S. Pat. No. 3,360,599 taught the use of an annealing procedure. Disadvantageously, this annealing procedure requires elevated temperatures and/or long annealing times. As a consequence, the resulting films often have inferior strength properties, due to polymer degradation, and/or the time required for film formation is unacceptably long.
U.S. Pat. No. 4,199,490 teaches the addition of a second aqueous dispersion comprising a rubber, synthetic resin or a mixture thereof to enable the formation of films upon drying at room temperature. In the absence of such additive, the block copolymer dispersion did not possess adequate film-forming properties at moderate or low temperatures.
In U.S. Pat. 3,238,173, there was disclosed the preparation of concentrated aqueous dispersions by contacting the dilute latex with an aliphatic hydrocarbon that is a non-solvent for the non-elastomeric block, removing the hydrocarbon and concentrating the latex. The use of such non-solvents is undesirable, due to the added complexity of the process and the presence of residual organic contaminants in the resulting films.
Many block copolymers contain residual unsaturation in the conjugated diene block. Polymers containing residual unsaturation are susceptible to degradation due to exposure to ultraviolet light and/or ozone. In some applications, such degradation is unacceptable.
Accordingly, there remains a need to provide films prepared from aqueous dispersions of block copolymers having improved strength properties. In addition, it would be desirable to provide a process capable of preparing strong films from aqueous latices of block copolymers that uses relatively short times and mild temperature conditions for the annealing step to thereby avoid significant polymer degradation. It would be desirable to provide a process for the preparation of thin elastomeric articles by film deposition from a block copolymer latex that avoids the use or reduces the amount of additives. It is desirable to prepare films which have good stability in the presence of heat, ultraviolet light and ozone.